1. Field of the Invention
The present invention relates to a molding method and apparatus using a film sheet as a molding material, and more particularly to a molding method and apparatus for molding a deep drawing product with two walls spaced by a small gap.
2. Related Background Art
The present applicant has proposed a molding method and apparatus for molding an acoustic diaphragm by using a poly-para-phenyleneterephthalamide (hereinafter called PPTA) film suitable for deep drawing and excellent in acoustic physical properties (Japanese Patent Application No.5-107368).
This method and apparatus will be explained with reference to FIGS.9 to 12. A PPTA gel film is prepared which contains water at least 50% or more, or preferably 80% or more, as a swelling substance and has a thickness of about 140 .mu.m. The swelling water is evaporated in an oven at about 180.degree. C. to obtain an amorphous hard film F which is not drawn and has a density lower than a predetermined value. This film is pulled and placed between upper and lower metal molds as shown in FIG.9. The upper metal mold 1u is heated to 380.degree. C. and the lower metal mold 1b is heated to 250.degree. C. Under this condition, the film is pressed for about 30 seconds and thermally fixed. Thereafter, a changeover valve 3 of the upper metal mold 1u is switched to the side of an exhauster 5. While the exhauster 5 is operated, a cylinder S is raised and the metal molds are released. Thereafter, an exhauster 5 of the lower metal mold 1b is stopped and a valve 3 is switched to the exhauster 5 to operate it, and the thermally molded product is removed from the metal molds.
This molding method and apparatus are suitable for molding a product having a relatively simple shape, for example, an ordinary dome-shaped of cone-shaped loudspeaker diaphragm, as shown in FIG. 9. However, with this molding method, it is difficult to remove a molded product smoothly from the metal molds, if the product has a complicated shape such as shown in FIG.10. This diaphragm has a cone C, a voice coil bobbin bo, and a chamber Cb integrally molded. A voice coil VC is inserted into the bobbin bo having a slit with a gap I of about 0.3 to 0.2 mm. This product is a deep drawing product having a plurality of vertical portions (bobbin bo) spaced by a very small gap.
A film molding apparatus generally uses film pushing rings 6u and 6b depending on the position relationship between a film and metal molds and the shape, material, and the like of a product to be molded. In the conventional apparatus shown in FIG.11, the rings 6u and 6b are mounted on the upper and lower metal molds 1u and 1b by springs Sp, and they move as the metal molds are moved up and down.
This motion will be detailed with reference to FIGS.11 and 12. While the metal molds 1u and 1b are being released, the lower ring 6b supports the film F pulled to an intermediate position between the upper and lower molds 1u and 1b which is a predetermined position not allowing the film F to contact the lower metal mold 1b. As the upper metal mold 1u lowers, the upper ring 6u lowers. As shown, the upper and lower rings 6u and 6b are disposed projecting from the upper and lower metal molds 1u and 1b. Therefore, as the upper metal mold 1u lowers, the upper ring 6u and lower ring 6b abut together to clamp the film F before the upper metal mold 1u abuts the lower metal mold 1b.
As the upper metal mold 1u further lowers, the springs Sp mounted on the rings 6u and 6b are compressed and the rings 6u and 6b clamping the film F lower. Thereafter, the clamped film F is pressed by the upper and lower metal molds 1u and 1b. As shown in FIG.11, during this pressing, the rings 6u and 6b are near at the intermediate position of the outer walls of the metal molds 1u and 1b pressing the film F. Since the metal molds 1u and 1b are being heated to predetermined temperatures by heater blocks 2, the film F is thermally molded. After the film F is pressed for a predetermined time, the upper mold rises and released from the lower mold. As the compression forces of the springs Sp become weak, the rings 6u and 6b start rising to retain the initial position. As the upper ring 6u further rises toward the upper metal mold 1u, the upper metal mold 1u detaches from the unmolded outer circumference of the molded product, i.e., the film F.
The motion of the rings 6u and 6b described above poses no practical problem so long as the shape of a product is simple. However, in the case of a product having a complicated shape such as a plurality of vertical portions described above, the vertical portions, particularly those of the bobbin bo, rise in tight contact with the upper metal mold 1u, resulting in a defective product.
In order to deal with the above disadvantages, the present applicant has proposed a molding method whereby a product is attracted and attached to the upper metal mold 1u or lower metal mold 1b to remove it from the metal molds. If a product is attracted and attached to the lower metal mold 1b, it can be removed smoothly without a tight contact of the upper metal mold 1u with the vertical portions, when the metal molds 1u and 1b are released (when the upper metal mold 1u rises).
However, if this approach is used with the conventional apparatus in which the motion of the rings 6u and 6b follows the motion of the metal molds 1u and 1b, the rings 6u and 6b start moving when the upper metal mold 1u is completely removed from the lower metal mold 1b. In this state, the rings 6u and 6b move while clamping the film F. Therefore, as shown in FIG.12, the outer circumference of the product is raised by the rings 6u and 6b, resulting in a defective product.
On the other hand, if a product is attracted and attached to the upper metal mold 1u, a defective product is formed which is similar to a product formed by using an approach of not attracting and attaching the product to the metal molds 1u and 1b. In order to deal with this phenomenon, we have tried to remove a produce by blowing air from suction holes formed in the metal molds 1u and 1b immediately before the rings 6u and 6b are moved. This approach is very effective for a product having a simple shape. However, in the case of a product having a plurality of vertical portions described above, a defective product is formed by a local deformation, for example, a deformation of the cone C although the chamber Cb is normal.
Specifically, a product with a plurality of vertical portions has a plurality of partial products partitioned by the vertical portions. A local deformation is inevitable unless a uniform pressure is applied at the same time to each partial product constituting the chamber Cb and cone C. Although this phenomenon can be settled to some degree if the position, number, diameter, and the like of air blowing holes are devised, a defective occurrence factor becomes large in the case of mass production of molded products.
The reason for this is as follows. A tight contact state of a product to the metal molds 1u and 1b changes with time if there is a fine variation of thicknesses, materials, and the like of the film F. Therefore, air blow matching each tight contact state is required. However, it is difficult to detect a tight contact state and adjust an air pressure so as to follow a change in the tight contact state. This approach is therefore unable to be realized in practice. Furthermore, a constant air blow results in a defective product with a local deformation.
The rings 6u and 6b are very effective for pressing shrinks of the film F formed during the molding. It is necessary, however, to optimize the inner diameter shapes of the rings 6u and 6b, the clamping positions, and the like, depending on the shape of a product to be molded. However, the conventional apparatus has the rings 6u and 6b directly mounted on ring supports 8, and there is a small space near at the metal molds 1u and 1b being unable to use jigs. As a result, in replacing the rings 6u and 6b, the ring supports 8 are dismounted from the base side and the springs 6u and 6b are dismounted, complicating the replacement work. Mounting and dismounting the metal molds 1u and 1b and other works are possible only after the rings 6u and 6b are dismounted, complicating the maintenance work.